Digital cellular telephones permit telephonic communication between users thereof. Digital cellular telephones employ a digital receiver that receives radio frequency (RF) signals and converts the RF signals to extract a data signal. The data signal is supplied to a vocoder which decodes the data signal, generates a speech signal therefrom, and outputs the speech signal for use by a user. RF signals may become corrupted prior to, and during, reception. Corrupted RF signals may result in a corrupted data signal and a severely degraded speech signal.
The digital data signal extracted by the digital receiver, by its nature, lacks background noise unlike that present in traditional analog receivers. As such, a traditional digital receiver does not produce noise in association with a corrupted speech signal. Traditional digital receivers alert the user to corrupted speech by letting the user hear distorted speech, issuing a special bad reception tone, or muting the speech. In these instances, the user can not tell if telephonic communication has ended, the receiver has become inoperative, the antenna of the telephone needs reorienting, or the like.
A better approach to alerting the user of a digital receiver to corrupted speech is disclosed in U.S. Pat. No. 5,327,457 by Leopold, assigned to Motorola Inc. In U.S. Pat. No. 5,327,457, a background noise signal is generated based on a low signal level of received RF signals. The background noise signal is added to the speech signal to improve user ambiance by making the digital receiver sound more like analog receivers with which users are more familiar. However, signal level is not always the best indicator that speech will be corrupted. For example, interference can cause corruption in receive RF signals without reducing signal level.
Therefore, it would be beneficial to carry the concept of U.S. Pat. No. 5,327,457 further by providing an improved apparatus and method for generating noise in a digital receiver.